U.S. patent number 10,319,553 [Application Number 16/084,433] was granted by the patent office on 2019-06-11 for method for controllably growing zno nanowires.
This patent grant is currently assigned to Lightlab Sweden AB. The grantee listed for this patent is Lightlab Sweden AB. Invention is credited to Jan-Otto Carlsson, Patrik Hollman, Helena Tenerz, Jonas Tiren.
United States Patent |
10,319,553 |
Tiren , et al. |
June 11, 2019 |
Method for controllably growing ZnO Nanowires
Abstract
The present invention relates to a method for controllably
growing ZnO nanowires, for example to be used in relation to field
emission lighting. In particular, the invention relates to a method
of controlling thermal oxidation conditions to achieve steady-state
conditions between an oxygen consumption rate by a growing oxide on
a surface of a structure and the decomposition rate of the
oxygen-carrying species within the chamber. The invention also
relates to a corresponding field emission cathode.
Inventors: |
Tiren; Jonas (Uppsala,
SE), Carlsson; Jan-Otto (Uppsala, SE),
Tenerz; Helena (Uppsala, SE), Hollman; Patrik
(Uppsala, SE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Lightlab Sweden AB |
Uppsala |
N/A |
SE |
|
|
Assignee: |
Lightlab Sweden AB (Uppsala,
SE)
|
Family
ID: |
59851596 |
Appl.
No.: |
16/084,433 |
Filed: |
March 14, 2017 |
PCT
Filed: |
March 14, 2017 |
PCT No.: |
PCT/SE2017/050246 |
371(c)(1),(2),(4) Date: |
September 12, 2018 |
PCT
Pub. No.: |
WO2017/160212 |
PCT
Pub. Date: |
September 21, 2017 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
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US 20190080869 A1 |
Mar 14, 2019 |
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Foreign Application Priority Data
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|
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Mar 16, 2016 [SE] |
|
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1650356 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J
9/025 (20130101); H01L 21/02554 (20130101); H01L
21/02614 (20130101); H01L 21/02603 (20130101); H01J
2201/30415 (20130101); B82Y 30/00 (20130101); H01J
2201/30446 (20130101); H01J 63/06 (20130101) |
Current International
Class: |
H01J
9/02 (20060101); H01L 21/02 (20060101); B82Y
30/00 (20110101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1709665 |
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Oct 2006 |
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EP |
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20050005122 |
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Jan 2005 |
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KR |
|
Other References
International Search Report and Written Opinion for International
Application No. PCT/SE2017/050246 dated May 30, 2017, 10 pages.
cited by applicant.
|
Primary Examiner: Hines; Annie M
Attorney, Agent or Firm: RMCK Law Group, PLC
Claims
The invention claimed is:
1. A method for controllably growing zinc oxide (ZnO) nanowires on
a surface of a structure by means of thermal oxidation, the
structure comprising a zinc layer covering at least a portion of
the structure, the method comprising: arranging the structure
within a thermal oxidation chamber, the chamber having a gas inlet
and a gas outlet for allowing a gas flow through the chamber;
providing a gas comprising an oxygen-carrying precursor through the
gas inlet of the chamber; and controlling a concentration of oxygen
along the surface of the structure by: controlling a temperature
within the chamber; and controlling a gas flow of the gas
comprising the oxygen-carrying precursor through the chamber, such
that steady-state conditions are achieved between an oxygen
consumption rate by a growing oxide on the surface of the structure
and the decomposition rate of the oxygen-carrying species within
the chamber, thereby maintaining the same zinc oxidation conditions
along the surface of the structure within the chamber.
2. The method according to claim 1, wherein said gas comprises a
plurality of oxygen carrying precursors.
3. The method according to claim 1, further comprising controlling
a gas pressure to provide substantially uniform growth conditions
at the entire surface of the structure, at a given time.
4. The method according to claim 1, further comprising controlling
the gas flow such that a resulting concentration of oxygen is
substantially uniform for the entire surface of the structure.
5. The method according to claim 1, wherein the oxygen-carrying
precursor is selected from a group comprising of O.sub.2, CO.sub.2,
N.sub.2O and H.sub.2O.
6. The method according to claim 1, further comprising selecting a
concentration of the oxygen precursor of the gas provided to the
chamber.
7. The method according to claim 1, wherein the gas is a gas
mixture further comprising at least one of nitrogen and argon.
8. The method according to claim 1, wherein the temperature is
controlled according to a predetermined temperature curve.
9. The method according to claim 8, wherein the temperature curve
is selected based on a decomposition rate of the oxygen-carrying
precursor.
10. The method according to claim 8, wherein the temperature curve
comprises ramping up the temperature to an oxidation temperature
using a fixed ramp, maintaining the oxidation temperature for a
predetermined time, and ramping down the temperature using a fixed
ramp.
11. The method according to claim 8, wherein the temperature curve
comprises: ramping up the temperature to a first oxidation
temperature using a first ramp rate; performing thermal oxidation
starting at the first oxidation temperature for a first period of
time to form an initial oxide layer; ramping up the temperature to
a second oxidation temperature using a second ramp rate; performing
thermal oxidation at the second oxidation temperature for a
predetermined period of time to initialize and to maintain nanowire
growth; ramping up the temperature to a third oxidation temperature
using a third ramp rate and performing thermal oxidation during the
temperature ramp to grow nanowires; and when the third oxidation
temperature is reached, ramping down the temperature to end the
oxidation using a fourth ramp rate.
12. The method according to claim 11, further comprising, when the
third oxidation temperature is reached, ramping the temperature up
or down to a fourth temperature using a fourth temperature ramp,
and maintaining the fourth temperature for a predetermined period
of time, before ramping down the temperature to end the oxidation
using a fifth ramp rate.
13. The method according to claim 1, wherein the third oxidation
temperature within the chamber is equal to or lower than
625.degree. C.
14. The method according to claim 1, wherein a pressure within the
chamber is a maximum of 1 atm.
15. The method according to claim 1, further comprising: preparing
the structure by applying a predetermined thickness of a ZnO layer
to the surface of the structure.
16. The method according to claim 1, wherein the structure
comprises at least one of copper and brass.
17. The method according to claim 1, wherein the structure
comprises a wire, a mesh or a plate.
18. The method according to claim 1, wherein a length of the ZnO
nanowires is selected to be between 2-100 um.
19. The method according to claim 1, wherein a diameter of the ZnO
nanowires is selected to be between 5-100 nm.
20. A field emission light source comprising a structure provided
with ZnO nanowires grown according to claim 1.
21. The field emission light source according to claim 19, wherein
the field emission light source is a UV light source.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a 371 U.S. National Stage of International
Application No. PCT/SE2017/050246, filed Mar. 14, 2017, which
claims priority to Swedish Application No. 1650356-7, filed on Mar.
16, 2016. The disclosures of each of the above applications are
incorporated herein by reference in their entirety.
TECHNICAL FIELD
The present invention relates to a method for controllably growing
ZnO nanowires, for example to be used in relation to field emission
lighting. The invention also relates to a corresponding field
emission cathode.
BACKGROUND OF THE INVENTION
Field Emission Light sources (FEL) are of interest as an
alternative to LED technology. FEL technology is environmentally
friendly, avoids the large blue portion of the visible spectrum as
used in LEDs and can be far more energy efficient as compared to
LEDs in the UVC region.
Field emission is a phenomenon which occurs when a very high
electric field is applied to the surface of material. This field
will give electrons enough energy such that the electrons are
emitted from the material.
In prior art devices, a cathode is arranged in an evacuated
chamber, having for example glass walls, wherein the chamber on its
inside is coated with an electrically conductive anode layer.
Furthermore, a light emitting layer is deposited on the anode. When
a high enough potential difference is applied between the cathode
and the anode thereby creating high enough electrical field
strength, electrons are emitted from the cathode and accelerated
towards the anode. As the electrons strike the light emitting
layer, typically comprising a light powder, the light powder will
emit photons. This process is referred to as
cathodoluminescence.
One example of a light source applying field emission light source
technology is disclosed in EP1709665. EP1709665 disclose a bulb
shaped light source comprising a centrally arranged field emission
cathode, further comprising an anode layer arranged on an inside
surface of a glass bulb enclosing the field emission cathode. The
disclosed field emission light source allows for omnidirectional
emission of light, for example useful in relation to a retrofit
light source implementation. In addition tube forms are also of
interest. This requires relatively long cathodes to be
manufactured. Other shapes are possible such as flat lamps.
Several different materials may be used to create the necessary
nanostructures used in order to achieve the extremely high
electrical fields needed to operate a field emission light source
at reasonable applied voltages. Carbon Nano Tubes (CNT's) have been
used extensively to demonstrate the technology. However CNTs may
erode during operation and carbon is deposited onto the anode, and
will degrade the performance. CVD based nano-diamond films are also
used, and work well, but these require long processes and expensive
equipment and are therefore expensive to make.
An alternative is to use zinc oxide (ZnO). This material shows very
little degradation, is inexpensive, and nanostructures may be
created in several ways.
ZnO has excellent electron emission properties, particularly from
rod or wire nanostructures. A cheap and attractive fabrication
technique is to simply oxidize zinc metal or a zinc-carrying alloy
in an oxygen-carrying atmosphere at elevated temperatures.
In order for the light source to operate with a uniform emission of
photons over a relatively large area, the light powder must be
uniformly bombarded by electrons and thus the uniformity of the
electron emitting nanostructure properties must be controlled over
a relatively large cathode area. In addition, for the manufacturing
of such cathodes the uniformity must also apply when manufacturing
large amounts of cathodes at the same time, i.e. all cathodes must
be relatively equal. These processes may take several hours in
process time, so when manufacturing high volumes, the cathodes must
be manufactured in large quantities at the same time (also referred
to as "batch manufacturing").
In order to achieve a commercially attractive product, the
properties of nanostructured elements must therefore be well
controlled over large areas and it must be possible to manufacture
large numbers of cathodes at the same time. Such considerations of
uniformity over large areas or lengths as well as uniformity in
larger reaction chambers are not found in present literature.
The present invention describes a highly uniform and reproducible
zinc oxidation process yielding long ZnO nanowires with excellent
and stable electron emission properties over long periods of
time.
SUMMARY OF THE INVENTION
In view of above-mentioned and other drawbacks of the prior art,
and the desired properties of a zinc oxidation process, it is an
object of the present invention to provide an improved method of
growing ZnO nanowires.
According to an aspect of the invention, the above object is at
least partially achieved by a method for controllably growing zinc
oxide nanowires on a surface of a structure by means of thermal
oxidation, the structure comprising a zinc layer covering at least
a portion of the structure, the method comprising: arranging the
structure within a thermal oxidation chamber, the chamber having a
gas inlet and a gas outlet for allowing a gas flow through the
chamber, providing a gas comprising an oxygen-carrying precursor
through the gas inlet of the chamber; and controlling a
concentration of oxygen along the surface of the structure by
controlling a temperature within the chamber and controlling a gas
flow of the gas comprising the oxygen-carrying precursor through
the chamber such that steady-state conditions are achieved between
an oxygen consumption rate by a growing oxide on the surface of the
structure and the decomposition rate of the oxygen-carrying species
within the chamber, thereby maintaining the same zinc oxidation
conditions along the surface of the structure within the
chamber.
In the present context, the term nanowire refers to a structure
where at least one dimension is on the order of up to a few
hundreds of nanometers. Such nanowires may also be referred to as
nanotubes, nanorods, nanopencils, nanospikes, nanoneedles, and
nanofibres.
Moreover, the structure serving as a base for nanowire growth may
be metallic, semiconducting or insulating, and may have any shape
and form. The structure may also be a layered or coated structure,
and the structure may be either mechanically rigid or flexible.
Example structures will be discussed in further detail in the
following.
A main objective of the manufacturing method according to various
embodiments of the invention is to provide an ensemble of ZnO
nanowires having sufficiently uniform properties to be used as
field emitters, for example in a lighting application. Field
emission occurs when a large enough electrical field is applied to
a material. For a flat surface, typical field strengths are in the
order of a few Gigavolt/meter. In practical applications these
voltages are far too high and therefore several steps are taken to
enhance the local field strength to achieve local field emission.
Using a tubular structure as an example, the first amplification
comes from the cylindrical symmetry where the electrical field E is
given by
.function..function. ##EQU00001## where V is the applied voltage, r
is the radius of the cathode and R is the radius of the anode
tube.
Using a typical example of R=15 mm and r=0.5 mm the resulting field
strengths at V=1000V becomes 0.59 MV/m, to be compared with a
corresponding flat structure with the same distance (14.5 mm)
giving a field strength of 0.069 MV/m.
The second step of the field amplification can be provided by using
nanostructures which provide extremely sharp tips that will enhance
the field further. The emission for a single emitter follows the
Fowler-Nordheim equation
.times..times..beta..times..0..times..times..times..0..times..times..beta-
..times..times. ##EQU00002## where A.sub.r is the effective emitter
area, a is the first Fowler-Nordheim constant;
.times..function. ##EQU00003## b is the second Fowler-Nordheim
constant;
.times..function..times..times. ##EQU00004## O is the work function
in eV (5.1-5.3 eV for ZnO) and .beta. is a dimensionless
enhancement factor. As long as the emitters are operating with
field emission, a plot of
.function..times..times..times. ##EQU00005## will give a straight
line, and .beta. can be found from the slope.
The enhancement factor .beta. will depend on the morphology of the
emitter. In a first order approximation .beta. will be depending on
the height h and the sharpness r of the nanostructure
.beta..varies. ##EQU00006##
In addition, the enhancement factor will be influenced by the
distance (d) to the next emitter.
Oxidation is a large scale method for manufacturing nanostructures,
which provides a good cost efficiency. However, when using an
oxidation method that will provide a very large number of
nanostructures it is obvious that the individual nanostructures
will not be exactly the same. The geometrical parameters h, r and d
will all have distributions and therefore .beta. will have a
distribution and the total current of all the emitters will
essentially be the integral of a distribution. This may be
expressed as
.0..times..times..times..beta..times..times..times..0..beta..times.
##EQU00007## where the subscript i denotes the specific properties
of each specific emitter, or in this case of each ZnO nanowire.
As the Fowler Nordheim equation above in practice is very steep,
only a portion of the distribution of nanowires will be
operational. When a field emitting device is turned on for the
first time, the (relatively few) emitters that turn on early may
become overloaded and destroyed due to e.g. ohmic heating. Once a
region of the distribution is reached where the emitters are large
enough to survive the applied field and the resulting emission
current, only those with appropriate characteristics will be
operating--in essence those with lower .beta..sub.i will not emit
any current of relevance.
The above means that the geometric distributions of the nanowire
properties must be controlled in order to ensure there are roughly
the same amount of emitters per unit area with similar
characteristics within the operational range over the entire
operational area of the cathode, and also that the properties can
be repeatable between process runs. Accordingly, it is therefore of
high importance to control the distribution of the nanowire
properties.
During thermal oxidation, many metals and metal alloys first form a
surface oxide layer having a morphology that may change with the
oxidation conditions (temperature and oxygen activity at the
metal/atmosphere interface). Upon increasing the oxidation
temperature a sequence of oxide layer structures is often obtained.
Starting with a grain structure, covering the metal or alloy
surface, a transition to whisker or surface wire structure can be
observed. At even higher temperatures the oxide surface morphology
becomes flatter and the surface wires/rods/whiskers disappear. This
means there is a certain oxidation temperature range allowing for
growth for zinc oxide nanowires. The dimensions of the
whiskers/wires depend on the oxidation conditions and the
properties of the underlying zinc layer but also on the inherent
properties of the oxide.
Zinc oxide is an n-type semiconductor with vacancies in the
Zn.sup.2+ ion positions. After growth of the initial oxide layer,
covering the Zn-surface with a thin oxide layer, the continued
oxidation process proceeds by means of Zn.sup.2+ diffusion via
Zn.sup.2+ vacancies through the oxide layer and/or via oxide grain
boundaries, i.e., the growth of the oxide occurs at the
oxide/atmosphere interface. The zinc oxidation rate is basically
determined by the supply rate of zinc through the oxide layer to
the zinc/atmosphere interface, the supply rate of oxygen-carrying
species or the chemical reaction rates of or between species. For
high supply rates of oxygen or oxygen-carrying species in
comparison with the supply rate of zinc to the oxide/atmosphere
interface, i.e. when the oxidation process is not oxygen limited,
the oxide layer can be assumed to grow externally while maintaining
a relatively flat surface structure. However, when the oxygen
supply rate is limiting the reactions, i.e. in an oxygen limited
reaction, the oxide growth will occur in a concentration gradient
developed in the atmosphere perpendicularly to the metal surface
i.e., a more instable growth front has been established and
conditions for nanowire growth has been reached. In summary the
interaction between two opposite fluxes (zinc and oxygen,
respectively) determines to a large extent the surface structure of
the formed oxide. The zinc flux is basically determined by the
temperature while the oxygen flux is determined not only by the
temperature but also by the gas flow rate, concentrations/partial
pressures of the gaseous species, the total pressure and the
chemical kinetics producing the oxidizing species, i.e. the
decomposition of an oxygen-carrying precursor.
The oxygen activity at the oxide/atmosphere interface during the
growth of the nanowires determines also the electron work function
of the material and hence the electron emission properties.
Moreover, the resistivity of the nanowires will also be affected by
the oxidation process. Zinc oxide is a non-stoichiometric n-type
semiconductor whose chemical composition is determined by the
oxygen activity in the vapor at the zinc oxide/vapor interface.
Many different oxygen-carrying precursors can be used to oxidize
zinc metal. However, in order to establish an oxygen concentration
gradient close to the substrate, favoring nanowire growth, oxygen
donor reactions are preferred. Oxygen-donor reactions allow a
precise control of the oxygen concentration or pressure in the
oxidation chamber even at very low levels.
The oxidation process described herein may include oxygen donor
reactions like CO.sub.2.fwdarw.CO+O N.sub.2O.fwdarw.N.sub.2+O
H.sub.2O.fwdarw.H+OH.fwdarw.H.sub.2+O
In order to favor the donor reactions and to control the
homogeneous reactions in the vapor, i.e., the oxygen supply rate to
the growth interface, a hot-wall oxidation reactor is preferably
used. The use of a hot-wall reactor also facilitates scaling up of
the oxidation process.
Even if the oxidation process includes a complicated interplay
between various species and transport processes in a solid material
(the oxide) and in the vapor, only two stages in the process will
be discussed below. As described above the initial formation of the
covering zinc oxide layer is crucial and paves the way for the
later formation of the nanowires. By controlling the thickness of
the primary formed oxide layer, a transition from a flat oxide
grain structure to a nanowire growth mode can be induced. Since the
initial formation of the oxide layer is important for the
subsequent nanowire growth, the oxide layer and nanowire formation
steps are favorably controlled by using different temperatures in
the steps. This means that the ramping of the temperature must be
controlled in order to be able to grow nanowires of desired
dimensions (lengths and diameters) and properties.
From the above it is clear that, in order to achieve a stable, high
volume scalable nanostructure manufacturing process form
manufacturing field emitters that will be suitable for large area
cathodes the geometric distribution properties of the nanostructure
must be controlled, and in order to achieve this, the partial
pressures of the involved chemical gaseous species is preferably
controlled accurately over a large volume in the oxidation
chamber.
Therefore, the use of oxygen-carrying precursors that will
thermally decompose slowly and controllably may be used and a
balance between the decomposition rate, the gas flow, and other gas
dynamics may be established and the aforementioned gradients close
to the oxidation surface may be controlled, yielding the desired
geometrical distributions.
The present invention is based on the understanding that ZnO
nanowires having uniform properties can be achieved by performing
thermal oxidation and controlling the oxidation process to produce
steady-state conditions between oxygen consumption rate by the
growing oxide surface and the decomposition rate of oxygen-carrying
species within the oxidation chamber. In the present context,
steady state conditions can be considered to mean that the
consumption rate of oxygen is matched by the decomposition rate
such that the oxidation process is substantially uniform, in the
reactor volume where oxidation takes place, over time throughout
the oxidation process. Accordingly, the process parameters are
controlled to achieve the above steady state, i.e. to provide the
correct amount of the oxygen-carrying precursor, at the appropriate
temperature and pressure, so that the oxidation process occurs at
the same conditions in the axial direction throughout the reaction
chamber over time. This in turn results in ZnO nanowires having a
high degree of uniformity over the surface of the substrate as well
as a high degree of uniformity between different substrates within
the reactor volume.
According to one embodiment of the invention, the temperature and
the gas-flow may be controlled to provide substantially uniform
growth conditions at the entire surface of the structure, at a
given time. To achieve uniform growth conditions, at a given point
in time, it is preferable that the relation between Zn and O at the
surface is substantially the same throughout the oxidation chamber,
or at least over the surface of the structure to be oxidized. This
can be achieved by controlling the gas flow of the gas containing
the oxygen-carrying precursor and the temperature in the oxidation
chamber based on known properties of the chamber.
According to one embodiment of the invention, the supplied gas may
advantageously comprise a plurality of different oxygen carrying
precursors. Moreover, a plurality of different oxygen carrying
precursors may be provided by means of one or more different
gasses.
According to one embodiment of the invention, the method may
further comprise controlling a gas pressure to provide
substantially uniform growth conditions at the entire surface of
the structure, at a given time.
According to one embodiment of the invention, the method may
further comprise controlling a gas flow such that a resulting
concentration of oxygen is substantially uniform over the entire
surface of the structure.
The influence that the gas pressure and gas flow has on the ZnO
nanowire growth will be discussed in the following detailed
description.
According to various embodiments of the invention, at least one
oxygen-carrying precursor may be selected from a group comprising
of O.sub.2, CO.sub.2, N.sub.2O and H.sub.2O. The different
oxygen-carrying precursors may have different decomposition rates,
in turn leading to different concentrations of free oxygen
available for the oxidation process. It should also be noted that
the decomposition rate typically is temperature dependent.
Accordingly, the choice of oxygen-precursor may be based on a
particular substrate and on the desired resulting properties of the
ZnO nanowires. In other words, the achievable range of the oxygen
concentration in the chamber can be controlled by selecting the
appropriate oxygen-precursor, where a selected oxygen-precursor can
provide a range of oxygen concentrations. The specific oxygen
concentration within that range is determined by process parameters
such as temperature and pressure.
According to one embodiment of the invention, the method may
further comprise selecting a concentration of the oxygen precursor
of the gas provided to the chamber. In addition to selecting which
oxygen-carrying precursor to use as discussed above, the oxygen
concentration in the chamber can be further controlled by
controlling the concentration of the oxygen-carrying precursor. The
gas provided to the oxidation chamber may for example be a gas
mixture comprising at least one of nitrogen and argon. However,
other inert gases may also be used in the gas mixture. Thereby, by
controlling the mix of the gas provided to the oxidation chamber,
the oxygen concentration can be controlled continuously during the
process, for example to be adapted to different growth stages of
the ZnO nanowires.
According to one embodiment of the invention, the temperature is
controlled according to a predetermined temperature curve, where
temperature curve is referring to a predetermined temperature as a
function of time. Thereby, a repeatable and stable oxidation
process is provided which is capable of producing a large number of
ZnO nanowires, and which can be consistently repeated for a large
number of separate process runs.
In one embodiment, the temperature curve may be selected based on a
decomposition rate of the oxygen-carrying precursor. The
temperature curve may also be tuned to other process parameters
such as gas flow and gas pressure, together providing a
manufacturing method which provides ZnO nanowires having the
desired uniformity.
According to one embodiment of the invention, the temperature curve
may comprise ramping up the temperature to an oxidation temperature
using a fixed ramp, maintaining the oxidation temperature for a
predetermined time, and ramping down the temperature using a fixed
ramp. Thereby, a steady state ramp over a predetermined period of
time may be used for ramping up and down the temperature to and
from the constant oxidation temperature.
According to one embodiment of the invention, the temperature curve
may comprise: ramping up the temperature to a first oxidation
temperature using a first ramp rate, performing thermal oxidation
starting at the first oxidation temperature for a first period of
time to form an initial oxide layer, ramping up the temperature to
a second oxidation temperature using a second ramp rate, performing
thermal oxidation at the second oxidation temperature for a
predetermined period of time to initialize and maintain nanowire
growth, ramping up the temperature to a third oxidation temperature
using a third ramp rate and performing thermal oxidation during the
temperature ramp to grow nanowires; and when the third oxidation
temperature is reached, ramping down the temperature to end the
oxidation using a fourth ramp rate.
Moreover, the temperature curve may comprise a fourth stage, after
the third stage. During the fourth stage, the surface properties of
the ZnO nanostructures may be modified and controlled by
introducing for example a small amount of water vapor, and/or a
doping agent, such as Al. By doping the ZnO nanostructures, the
field emission properties can be improved. In this embodiment, the
temperature is ramped down or up from the third temperature to a
fourth temperature, using a fourth temperature ramp. The fourth
temperature is then held constant for a preselected period of time
after which the temperature is ramped down to room temperature
using a final temperature ramp.
Thereby, the temperature curve defines the three main stages of the
ZnO nanowire growth: the first stage forming a substantially planar
initial oxide layer forming a base for subsequent nanowire growth,
the second stage initializing the nanowire growth, and the third
stage where the ZnO nanowires are grown and the length of the
nanowires are grown and determined. The second stage can be
referred to as a nucleation and growth stage of the nanowires,
where the population density of ZnO nanowires can be controlled by
controlling the relevant process parameters. Moreover, for a
constant temperature and a constant oxygen concentration, the
growth process is in principle self-limiting as a result of reduced
Zn diffusion to the oxide/atmosphere interface with increasing
oxide thickness. Accordingly, the temperature is increased during
the third stage to facilitate continued nanowire growth, thereby
controlling the final length of the nanowires.
According to one embodiment of the invention, the third oxidation
temperature within the chamber may advantageously be equal to or
lower than 625.degree. C. It has been found that ZnO nanowires
having desirable uniformity and emission properties can be
manufactured when the highest oxidation temperature does not exceed
625.degree. C. A too high oxidation temperature also limits the
range of possible substrate materials.
According to one embodiment of the invention, a pressure within the
chamber may advantageously be at maximum 1 atm. By performing the
oxidation at a pressure not exceeding 1 atm, there is no need for
the specific pump equipment and/or oxidation chamber required for
operating at elevated pressure. Thereby, the manufacturing process
is relatively simple and straightforward, and the repeatability of
the process may also be increased due to the relative ease of
maintaining the same conditions for subsequent runs of the
process.
According to one embodiment of the invention, the method may
further comprise preparing the structure by applying a
predetermined thickness of a ZnO layer to the surface of the
structure. In some cases, and for some substrate materials, is may
be advantageous to apply a ZnO layer prior to the start of the
thermal oxidation process. Such an oxide layer can for example be
deposited using evaporation, sputtering, or chemical oxidation in a
solution.
According to one embodiment of the invention, the structure may
comprise at least one of copper and brass, and the structure may
for example comprise a wire. The structure may also be a planar
substrate such a s a plate. The structure may additionally be
comprised of a wire mesh.
According to one embodiment of the invention, the method may
comprise preparing the structure by electroplating the structure.
An electroplated layer may serve as a seed layer for facilitation
subsequent ZnO nanowire growth. The electroplated layer may
advantageously be a zinc layer.
According to one embodiment of the invention, a length of the ZnO
nanowires may be selected to be between 2-100 um and a diameter of
the ZnO nanowires may be selected to be between 2-100 nm.
According to another aspect of the invention, there is also
provided a field emission light source comprising a structure
provided with ZnO nanowires grown according to any one of the
embodiments of the above described method. Furthermore, the field
emission light source may be a UV light source.
Further features of, and advantages with, the present invention
will become apparent when studying the appended claims and the
following description. The skilled addressee realize that different
features of the present invention may be combined to create
embodiments other than those described in the following, without
departing from the scope of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The various aspects of the invention, including its particular
features and advantages, will be readily understood from the
following detailed description and the accompanying drawings, in
which:
FIG. 1 is a flow chart outlining the general steps of a method
according to an embodiment of the invention;
FIG. 2 is a schematic illustration of an oxidation chamber for
performing a method according to various embodiments of the
invention; and
FIGS. 3A-B conceptually illustrate a temperature curves according
to embodiments of the invention.
DETAILED DESCRIPTION
The present invention will now be described more fully hereinafter
with reference to the accompanying drawings, in which currently
preferred embodiments of the invention are shown. This invention
may, however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided for thoroughness and completeness,
and fully convey the scope of the invention to the skilled
addressee. Like reference characters refer to like elements
throughout.
FIG. 1 is a flow chart outlining the general steps of the
invention, which will be described with reference to the thermal
oxidation chamber schematically illustrated in FIG. 2.
In a first step 102, a structure to be oxidized is placed in the
thermal oxidation chamber 202. The thermal oxidation chamber 202
may for example be a quartz tube of a hot-wall furnace, where
heating elements 204 arranged outside of the chamber heat the
environment within the chamber 202. A temperature controller 205
controls the heating elements 204 to provide a controlled
temperature within the oxidation chamber. In a hot-wall furnace,
the temperature can be uniform throughout the chamber where the
structures 206 are placed. The structures are here illustrated as
wafers 206 arranged in a carrier 208. Wafers are typically used as
substrates to enable large scale manufacturing in semiconductor
processes. However, the structures to be oxidized may also have
other shapes, such as wire-shaped. Moreover, the structures to be
oxidized may also be placed on carrier wafers to simplify
handling.
The oxidation chamber 202 comprises a gas inlet 210 and a gas
outlet 212 allowing a gas to flow through the chamber 202. To start
the oxidation, at least one oxygen carrying-precursor is provided
from one or more of the gas containers 214a-c. Throughout the
following description, it is to be understood that one ore more
oxygen-carrying precursors may be used. A gas container may also
comprise an inert gas for providing a gas mixture comprising an
oxygen-carrying precursor and an inert gas, thereby controlling the
oxygen-carrying precursor concentration in the chamber 202. The
oxygen-carrying precursor may be selected from the group comprising
O.sub.2, CO.sub.2, N.sub.2O and H.sub.2O, and the inert gas may be
N.sub.2. Each gas container 214a-c is connected first to a mass
flow controller (MFC) 211a-c for accurately controlling the gas
flow mixture into the oxidation chamber 202. A pump 213 is
connected to the outlet for pumping gas out of the oxidation
chamber 202.
Once the structures are placed in the oxidation chamber 202, a gas
comprising an oxygen-carrying precursor is provided 104 in the
chamber. The heating of the chamber 202 may be initiated prior to
providing the oxygen precursor. For example, an inert gas may be
provided to the chamber 202 while the chamber 202 and structures
206 are being heated to reach the desired oxidation
temperature.
After providing the oxygen precursor, the temperature 106 and the
gas flow 108 are controlled to produce steady-state conditions
between oxygen consumption rate by the growing oxide surface and
the decomposition rate of oxygen-carrying species within the
chamber. The consumption rate of oxygen varies over time throughout
the oxidation process. Therefore, it is important to control the
process and the decomposition of the oxygen-carrying precursor such
that the concentration of free oxygen is appropriate for the
different oxidation stages included in the overall process. Details
concerning the different stages of the oxidation process, as well
as the use of ZnO nanowires as field emitters, will be discussed in
the following.
FIG. 3A is an example temperature curve outlining the different
stages in an oxidation process. Prior to oxidation, the structures
to be oxidized are prepared. As an example, the base cathode
structure may be a Cu wire which is electroplated with zinc thereby
forming a zinc layer on the wire. Next, the prepared wire is
arranged in a holder and arranged in the oxidation chamber.
In a non-limiting example of the invention, the gas-flow related
process parameters of the thermal oxidation process may be
approximately, CO.sub.2=20 sccm, N.sub.2=400 sccm and the pressure
P(tot)=1 atm.
First the temperature is ramped up 302 from room temperature to the
first oxidation temperature 304, typically in the range of
350-450.degree. C. using a first ramp rate. At the first oxidation
temperature 304, the initial oxide layer is formed. After initial
oxide formation, the temperature is ramped up to a second oxidation
temperature 306, typically in the range of 500-550.degree. C.,
using a second ramp rate. Here, thermal oxidation is performed for
a predetermined period of time (2 h in the present example) to
initialize and continue to drive nanowire growth. Next the
temperature is slowly ramped up 308 to a third and final oxidation
temperature, typically in the range of 525-575.degree. C., using a
third ramp rate. In the present example, the temperature is ramped
over a period of about 5 h.
Once ZnO nanowire growth has been initiated, the growth rate
gradually slows down due to the increasing ZnO thickness, since Zn
must diffuse through the ZnO layer to the oxide/atmosphere
interface for reacting with oxygen. Therefore, the temperature is
slowly increased to increase the diffusion rate of Zn in order to
maintain the desired oxidation conditions.
An alternative temperature curve is illustrated in FIG. 3B, where a
fourth stage is added, comprising ramping down or up 312 the
temperature, after the third temperature has been reached, to a
fourth temperature 314 which is maintained constant for a
predetermined period of time before ramping down 310 to room
temperature. During the fourth stage, the surface properties of the
ZnO nanostructures may be modified, for example by introducing a
small amount of water vapor in the oxidation chamber and/or by
introducing a doping agent comprising for example Al.
Additionally it shall be noted that in some arrangements a simple
steady state ramp over a predetermined period of time may be
used.
In summary, the present invention relates to a method for
controllably growing highly uniform zinc oxide nanowires on a
surface of a structure by means of thermal oxidation.
By means of the invention it is possible to optimize the growth
process of ZnO nanowires to form field emitters having a high
uniformity.
It should be noted that the specific process parameters depends for
example on the properties of the oxidation chamber, the
oxygen-carrying precursor used, the geometry of the structures to
be oxidized and the desired properties of the resulting ZnO
nanowires. Due to the complex interplay between different
mechanisms, depending on a range of process parameters, the
specific process parameters are best determined empirically for a
given set of conditions and for a specific desired result.
Although the figures may show a specific order of method steps, the
order of the steps may differ from what is depicted. Also two or
more steps may be performed concurrently or with partial
concurrence. Such variation will depend on the software and
hardware systems chosen and on designer choice. All such variations
are within the scope of the disclosure. Likewise, software
implementations could be accomplished with standard programming
techniques with rule based logic and other logic to accomplish the
various connection steps, processing steps, comparison steps and
decision steps. Additionally, even though the invention has been
described with reference to specific exemplifying embodiments
thereof, many different alterations, modifications and the like
will become apparent for those skilled in the art.
Variations to the disclosed embodiments can be understood and
effected by the skilled addressee in practicing the claimed
invention, from a study of the drawings, the disclosure, and the
appended claims. Furthermore, in the claims, the word "comprising"
does not exclude other elements or steps, and the indefinite
article "a" or "an" does not exclude a plurality.
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